a=int (input())
p=0
for i in range(1,a+1):
    if i%3==0:
        p=p+1
print(p)
# a=int (input())
# p=[]
# n=0
# for i in range(1,a+1):
#     if i%3==0:
#         n=i
#         p.append(n)
# print(p)